Adobe Acrobat has a built in search feature within it allowing key word searches in large PDF files.
The search button in Acrobat (Binoculars)
Once viewing a PDF file, click on the binoculars icon to launch the search feature within Acrobat.
Key word search
Type in the phrase or key word that you are looking for. In this example it will look for the words "lead" and "shot" together and not individual occurrences of them.
Key word results page
The results screen will show how many occurrences were found. As the mouse is moved over the results, the page the word / words appear on is displayed. Click the occurrence in the search to be taken to that page, the word occurrence will then be highlighted on the page as in this example.